1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the moving velocity of a linear object such as a thread, metal wire or optical fiber, and more particularly to a method and an apparatus resorting to a technique for measuring the moving velocity of a linear object by illuminating the object with a laser beam to obtain a speckle pattern through a finite-sized aperture, converting the pattern to an electric signal and counting the number of zero-crossings per second of the signal after cutting off the d.c. component thereof.
2. Description of the Prior Art
Such a method of measuring the velocity of a moving object is described, for example, in Applied Physics, 25., 179-194(1981), particularly pp. 191-194, as the "Zero-Crossing Method." When an elliptic aperture or slit aperture is used as the finite-sized aperture, it is possible to detect not only the velocity of the moving object but also the direction of movement of the object.
However, the conventional method, in which a thin laser beam of circular cross section is used, has the following two drawbacks when measuring the velocity of a linear object which is moving in the direction of its length while vibrating.
(1) The circular spot of the laser beam is small and unable to illuminate the entire range of the amplitude of vibration of the linear object which is vibrating perpendicular to its length. If a large spot is used to cover the entire amplitude range, the laser beam has a reduced intensity (per unit area). Further because the laser beam produces a circular spot, the length of the illuminated portion of the vibrating linear object varies from position to position, making it impossible to afford accurate velocity measurements. PA1 (2) The velocity measured is the resultant of a velocity component in the longitudinal direction of the object and a vibration velocity component, so that it is impossible to measure the longitudinal velocity component alone accurately.